The present invention relates to servo control systems, and more particularly, to head positioning in servo control systems for disk drives.
Magnetic hard disk drives typically store user data blocks in data sectors arranged within concentric data tracks. The sectors typically include the user data and also include certain overhead information needed by the hard disk drive for head positioning and for locating user data within each data track. The overhead information is typically included within a servo region or sector header located ahead of each user data portion or sector. The servo sector may be recorded at the same frequency as the data sector, or the servo sector may be recorded at one data rate over the entire radial extent of the data storage surface. The user data sectors are typically recorded at data rates based on track radius which enables user data rates to be optimized at the particular radius. Radial zones of adjacent data tracks are frequently employed as a practical compromise between optimum data transfer rate and implementation complexity.
The servo information may include information for adjustment of read channel gain elements and block framing elements. The servo information may also include a track number field which is Gray-coded and used for coarse positioning during track seeking and settling operations, a sector number field for circumferential location information within the track. The servo information also typically includes track centering information, such as a plurality of radially and circumferentially offset bursts providing track centerline and quadrature reference information.
Many of the new disk drive systems with high track densities employ a head with a magneto-resistive (MR) read element formed in or over a thin film inductive write element. By using magneto-resistive sensing, a greater electrical output signal is achieved for a given flux change intensity from the recording surface. While MR readers provide greater sensitivity, they also typically operate non-linearly with response to absolute position over the flux transition. This non-linearity means that the MR read element may put out one signal level at a given off-track position in one radial direction of the storage disk, and another signal level at the same given off-track position in the opposite radial direction.
The read/write element separation in MR heads requires new off-track performance requirements in a disk drive. Furthermore, the MR read element is typically significantly narrower than the inductive write element in order to achieve better margins for tracking errors, for example. However, a narrow MR read element poses poor signal-to-noise ratio, and position error signal (PES) discontinuity and non-linearity problems for the servo system.
Many disk drives use both digital track number and multiple analog servo burst signals from disks to decide the position of a head using an MR read element. Depending on the head position within a track, the servo system decodes different pairs of servo bursts to determine the fractional track position. When switching from one pair of bursts to another, the servo system frequently perceives discontinuities in the head position due to the non-linearity of the MR head as a position transducer. These discontinuities become a serious servo problem when the destination/target head position/location is on one of said switching boundaries (switch points).
Some existing solutions have attempted to address this problem by printing different servo burst formats, or by utilizing different ways of decoding the positioning information, however, without solving the PES discontinuity and non-linearity problems at target head positions where servo burst switching occurs. What has been lacking in the prior approach has been a servo head positioning method and system that provides a substantially linear PES signal. There is also need for such a method and system to solve the tracking problems at the burst witching boundaries.
The present invention satisfies these needs. In one embodiment, the present invention provides a servo method and system for generating a position error signal in a disk drive including a magnetic data storage disk rotating relative to a base, a data transducer assembly positioned at concentric data tracks by a rotary actuator referenced to the base and controlled by a digital servo system, the data transducer assembly having a magnetoresistive (MR) read element, the digital servo system employing multiple servo bursts in servo sectors embedded within the data tracks. During servo track positioning/following for each target head position relative to a track, a plurality of the servo bursts are selected, and combinations of the burst amplitudes of the selected servo bursts are obtained. A substantially continuous position error signal (C_PES) is generated based on said combinations of the burst amplitudes, indicating radial position of the read head relative to the target position.
Selecting servo bursts includes selecting at least two pairs of servo bursts, and for each selected pair of servo bursts obtaining at least one combination of the burst amplitudes of the selected servo burst pair. Selecting servo bursts can be in accordance with predetermined burst phase switch points fixed in relation to radial position of the read head relative to a track. Obtaining a combination of the amplitudes of each servo burst pair includes the steps of differencing relative amplitudes of the servo burst pair to obtain a difference signal for each servo burst pair, thereby generating at least: (1) a first difference signal and (2) a second difference signal. To generate the C_PES signal, a combination of at least the first and the second difference signals is obtained, wherein the C_PES signal is based at least on that combination of the two difference signals. Combining the first and second difference signals includes the steps of generating a combination of at least the first and the second difference signals according to weighting functions. The weighting functions apply different weighting factors to each of the first and the second difference signals. The weighting factors can vary as function of target position.
As such, in one version, the present invention provides means of solving the conventional tracking problems at the burst switching boundaries by moving the switch points away from the destination/target position regardless of the location of said target position on disk, and provides continuous position error signal indicating position of the head relative to a target position without sudden changes in the C_PES signal.